The invention relates to an opto mechanical deflector with at least one rotating or slewing scan mirror element which is hit by a substantially parallel beam which is focused onto a flat image plane with a concave mirror near the flat image plane.
Such known deflectors can be used for various applications of flat bed scanning systems as input scanner or output scanners. One typical application is the scanning of a web of material in the deflection plane to inspect it for faults.
In the case at hand, one preferred application of the optical flat bed scanning system is the setting of typographic characters by means of a beam of light, in particular of a laser light source, modulated with a pixel frequency according to picture elements.
Generally, such optical flat bed scanning systems are to move a beam of light rays fixed in space in the direction of a scanning line across a planar deflection plane and, if at all possible, in such a manner that, normally, a deflection path increment of equal length corresponds to each pixel period, regardless of the location of the increment in the scanning line. In the optical flat bed scanning system of the kind mentioned at the outset, with a turnable or pivotable, reflecting surface, this includes also that a proportional distance in the deflection plane corresponds to each angle of rotation of this surface. In addition, the distances between consecutive scanning lines should be constant. This relation is restricted because of a number of possible errors referring among others, to the limited manufacturing accuracy of such flat bed scanning systems. In particular, the so-called wobble of the bearing in which the turnable or pivotable reflecting surface is mounted and the so-called polygon pyramidal error in the case of a polygon composed of several, mutually offset surfaces can disturb the relation of consecutive scanning lines (the polygon pyramidal error means that angular variation of a reflecting polygon surface each relative to a reference surface.)
In order to deflect scanning lines in a planar deflecting plane exactly despite such disturbing influences, which is especially desirable particularly in typographical applications because even the slightest irregularities are conspicuously disturbing there, it is self-suggesting to aspire to as large a deflection angle as possible. For, at a given deflection length, the optically effective distance or arm between the turnable image plane and the deflection plane can be correspondingly reduced by a greater deflection angle. This causes inaccuracies of motion and position of the reflecting surface, such as of a polygon, to have a lesser effect in the deflection plane.
In typical, known, optical flat bed scanning systems, the optic device to deflect a beam of light rays in deflection positions of the deflection plane essentially proportional to a deflection angle of the turnable reflecting surface, which optic is disposed between the turnable reflecting surface and the deflection plane, consists of flat field lenses in an arrangement of many members which is correspondingly expensive. These lenses are also called f.theta. lenses. But it is a disadvantage of such f.theta. lenses that they can be used only in a limited range of the deflection angle, especially when the resolution requirements are high as in a typesetter.